WOW!!!!!!!!I'm using NSUI beta 28 which has the option to customise the banner sound, I saw you said you also are using the same version. See the screenshot below, I've noticed in beta 27 this option is removed. Have you checked the version you are using? Silly question I know but this isn't my area of expertise.
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You're right, I only recently upgraded to beta 28 and I was using beta 27 before, so I didn't open "Customize" again thinking the option had been removed from all versions!! That's exactly what I needed, thank you!